The invention relates to a method and to apparatus for investigating geometrical characteristics of a borehole, and more particularly for detecting the distances between a logging sonde and the wall of a borehole in a plurality of directions around the sonde. The invention also relates to the application of such a technique to a logging method and apparatus for investigating the azimuthal resistivities of earth formations through which a borehole passes in order to correct said resistivity measurements.
Logging apparatuses are known that detect distances between a logging sonde and the wall of a borehole in a plurality of directions around the sonde. The purpose of such apparatuses is to measure a plurality of borehole diameters. An example of a mechanical implementation of such a multiple caliper tool is described in U.S. Pat. No. 4,251,921 (Fink). That apparatus comprises articulated arms adapted to come into contact with the wall of the borehole and a system for measuring the spread of the arms. That apparatus requires numerous moving mechanical parts that are liable to wear and that run the risk of jamming in difficult boreholes.
A different implementation of such a multiple caliper apparatus, based on acoustics, is described in U.S. Pat. No. 4,827,457 (Seeman et al.). That apparatus has the advantage of measuring a plurality of borehole diameters without contacting the borehole wall. It nevertheless requires a large number of acoustic sensors that are relatively expensive.
U.S. Pat. No. 4,087,740 (Suau) describes a logging apparatus having electrodes for determining the mean diameter of a borehole. In that apparatus, a first current is emitted which flows mainly into the formations, together with a second current that flows mainly in the borehole fluid. Two resistivity signals are derived in response to the first and second currents and the two resistivity signals are combined to deduce the mean diameter of the borehole. Such apparatus does not give information on the eccentring of the sonde in the borehole, nor does it give indication concerning the shape of the cross-section of the borehole.